Fluid heat exchangers are used to remove waste heat from high-heat flux heat sources (typically in excess of 5 watts/cm2, and often substantially higher) by accepting and dissipating thermal energy therefrom. Examples of such high-heat flux heat sources include microelectronics, such as microprocessors and memory devices, solid-state light emitting diodes (LEDs) and lasers, insulated-gate bipolar transistor (IGBT) devices, such as power supplies, photovoltaic cells, radioactive thermal generators and fuel rods, and internal combustion engines.
The fluid heat exchangers dissipate heat by thermally conducting the heat into internal passages of the exchanger, through which a coolant fluid flows, absorbing the heat conducted across the walls of the exchanger, and then transporting the fluid outside the exchanger, where the heat is rejected to an external heat sink. While the coolant fluid flowing through the exchanger may be a gas, it is generally preferable to use a liquid, as liquids have higher heat capacities and heat transfer coefficients than gases. The liquid may remain in a single phase, or the liquid may partially or completely evaporate within the internal passages of the exchanger.
The flow of coolant liquid fed to the fluid heat exchanger may be driven by a pump, or by natural convection due to density differences and/or elevation between the incoming and exiting fluid (e.g. thermosyphons), or by capillary action in the internal passages of the exchanger, or by a combination of these mechanisms.
Evaporator-type exchangers rely on the boiling mode, and have the advantages of higher heat transfer coefficients (better heat transfer) per unit of fluid flow rate of the coolant fluid. They also require much less coolant flow, as the majority of the heat is absorbed through via the latent heat of vaporization of the boiling fluid, rather than the sensible heat (heat capacity) of a single-phase liquid or gas.
It is well known that the thermal performance and efficiency of fluid heat exchangers can be greatly enhanced if the internal passages are comprised of microchannels, i.e. channels having cross-sections with a smallest dimension of less than 1000 microns, and more typically, in the range of 50-500 microns. However, as a consequence of the microchannel dimensions, the hydraulic diameters (D_h) of such passages are quite small and restrictive, resulting in high pressure drops (D_h=4×cross sectional area divided by the cross-sectional perimeter length). These high pressure drops tend to require more pumping power for pumped systems, and reduce the coolant flow rate for systems where the flow is driven by natural convection and/or capillary action. For pumped systems, this generally results in higher energy consumption (additional parasitic energy losses), while for natural-convection or capillary-driven systems, the reduced flows can result in reduced heat removal capacity or even dry-out of the fluid exchanger.
One way to reduce the pressure drop in the microchannel heat exchangers is to divide the flow path into shorter segments, i.e. splitting the flows, with the split segments being supplied from a common central inlet. This approach has been used in fluid heat exchangers configured to provide a split flow of a single-phase liquid in pumped systems.